1. Technical Field
The present disclosure relates to a microelectromechanical sensor with differentiated performances and to a method of controlling a microelectromechanical sensor.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has become increasingly widespread in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures comprising at least one mass coupled to a fixed body (stator) by springs and movable with respect to the stator according to pre-set degrees of freedom. The movable mass and the stator are capacitively coupled through a plurality of respective comb-fingered electrodes facing one another so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors, whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied. Conversely, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. Furthermore, to obtain electromechanical oscillators the frequency response of the inertial MEMS structures is exploited, which is typically of a second-order low-pass type, with a resonance frequency.
In particular, MEMS gyroscopes have a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and coupled together so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation at the resonance frequency. The other mass is drawn along in the oscillatory (translational or rotational) motion and, in the event of rotation of the microstructure with respect to a pre-set gyroscopic axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the mass drawn along, which is capacitively coupled to the fixed body through electrodes, as the driving mass, operates as an accelerometer that enables sensing of the Coriolis force and acceleration and hence tracing back to the angular velocity.
Notwithstanding the increasingly wide diffusion, the possibilities of exploiting MEMS inertial sensors are limited by a certain rigidity of use of the individual device. In particular, the performance in terms of sensitivity, scale, and noise rejection cannot be modified and hence each sensor can be used for a single application.
On the other hand, the need for a flexible single sensor is extremely felt in modern electronics. For instance, in portable electronic devices, especially in cell phones, numerous functions are based upon inertial sensors. More and more frequently, in fact, these devices include filming functions (image stabilization), display functions (orientation of images on the screen), game functions (where the device itself is used as controller), monitoring or emergency functions (sensing of free fall or impact), and auxiliary functions (pedometer), which are available thanks to the inertial sensors or in any case could benefit from of their use.
Each function, however, can have different types of performance. For instance, for image stabilization the devices should accurately detect very small movements, linked to the natural trembling of the user's hands. Instead, to sense impact that is potentially harmful for the device it is sufficient to recognize in a rather rough way that acceleration thresholds have been exceeded, and the influence of the noise is negligible. On the other hand, the time dedicated to image stabilization is generally limited and hence the power consumption is a secondary parameter. Many monitoring and emergency functions, instead, are performed continuously as long as the device is functioning. Consequently, to prevent severe limitation of autonomy, the reduction of the consumption levels is essential. Again, different types of performance can be used also by one and the same application. A videogame could, for example, privilege fine control of the movement in some steps and rapidity at the expense of the precision in others. Another field where different types of performance is used is the so-called enhanced reality, especially in applications dedicated to portable devices such as cell phones or palmtop computers. In this case, there is the need to sense macroscopic movements of the controller (for example, the cell phone itself) and, at the same time, perform functions of image stabilization.
It would hence be desirable to be able to use MEMS inertial sensors in a more flexible way.